Most semiconductor devices utilize several different levels of metallization. With the increasing complexity of devices and the need to reduce the physical size of devices, the number of levels which incorporate metal connections is increasing. In addition, with the desire to increase the speed of the devices while reducing the power consumed by the devices, advanced metallization schemes are being developed. One such scheme involves the use of copper-doped aluminum or copper structures for the bus lines and interconnects. Additionally, interlevel dielectrics with lower dielectric constants than standard silicon dioxide films may be used as the dielectric material situated between metallic structures.
A problem that most semiconductor manufacturers face is the cleaning up of the metallic structures after the structures are patterned and etched. More specifically, the photoresist needs to be removed, and the residual metal halide etch byproducts have to be removed or converted to different chemical forms to avoid corrosion of the metal. These processes, commonly known as strip and passivation processes, may cause non-conducting residues to form on the metallic structure. In order to address this problem, a cleaning step is typically performed after the metal structure is exposed and the photoresist is removed. The cleanup step will preferably remove all of the residue, typically comprised of inorganic polymers, that are formed on the metal structure, thus inhibiting corrosion of the metal structures. However, the clean step must not appreciably affect the electrical critical dimension (CD) of the metal structure.
Typically, for a metallization scheme consisting primarily of Al, the cleanup process consists of an HF vapor etch and an optional solvent clean. More specifically, an O2/H2O plasma photoresist strip and metal passivation step is performed followed by an ex situ HF vapor etch and solvent cleanup. The use of the separate HF vapor treatment equipment and the subsequent solvent “wet” clean increases processing costs and reduces production throughput. The use of a solvent is additionally undesirable due to environmental issues. Solvent recycling further increases the cost of the traditional cleanup process. In addition, both the HF vapor and solvent clean can cause CD loss, and may undercut the TiN/Ti barrier layers between the conductor and the dielectric substrate. Such cleanup processes may not sufficiently passivate the aluminum conductors, thereby resulting in the corrosion of the structure. Hence, a dry process which is environmentally friendly and which passivates the metal structure would be preferable. The use of an O2/H2O plasma passivation and strip step to remove the photoresist followed by an in situ O2/CF4 or H2O/CF4 plasma clean step to remove the polymeric residue from the metal structure has been used. However, the fluorine plasma may erode the process equipment in which this step is performed (e.g. any quartz parts in the process chamber would be eroded by the fluorine containing plasma).
For a typical Cu metallization scheme, a standard O2 plasma ash is performed to remove photoresist after a via etch process. Since ashing with O2 causes substantial oxidation to any exposed Cu at the bottom of the via, the via etch process generally completes to the Si3N4 barrier layer typically deposited to prevent Cu oxidation. The nitride layer must then be removed in a separate wet or dry etch process. Thus, a dry plasma etch process which could be used to remove photoresist without oxidation of Cu would simplify the process flow by either eliminating the need for the Si3N4 barrier layer, or substantially thinning it (it might still be useful as an etch stop layer for via formation). Removal or thinning of the Si3N4 barrier layer would ease the oxide etch selectivity requirements since stopping the etch on the Si3N4 layer would not be necessary.